Sturmanite , Ettringite Locality: N'Chwaning Mines, Kuruman , Kalahari manganese fields, Northern Cape Province , South Africa ( Locality at mindat.org ) A brilliantly yellow, lemon-colored, nearly-2-cm crystal is perched atop a knoll of matrix. The large crystal is sturmanite, and the smaller yellow specks in the matrix are probably ettringite - though often the same crystal may host both species in a transitional mix and its hard to say for sure without proper analysis. Surpassingly aesthetic! These came from a small series of finds, now some 20 years ago, and have not been found in this quality since. 4.9 x 4 x 3.6 cm — Photo: Robert M. Lavinsky · CC BY-SA 3.0
Sturmanite
A bright-yellow ettringite-group mineral with boron — long hexagonal prisms from the Kalahari manganese fields.
Intro adapted from Wikipedia (CC BY-SA).
Sturmanite is an IMA-recognized mineral species in the sulfate class, with the chemical formula Ca₆Fe³⁺₂(SO₄)₂.₅[B(OH)₄](OH)₁₂·25H₂O.
It crystallizes in the hexagonal crystal system (space group P31c). It is a member of the Ettringite structural group.
Key properties: Mohs hardness 2.5; specific gravity 1.847; Pale yellow, greenish yellow, brownish orange if altered streak; Vitreous, slightly greasy on fracture surfaces luster; optically uniaxial.
IMA symbol Strm, IMA status: approved, named in 1981.
Occurs as an uncommon secondary mineral in cavities in metamorphosed bedded manganese deposits.
It is found associated with Barite, manganite, hausmannite, hematite.
The mineral was named to honor Bozidar Darko Sturman (1937–), Canadian mineralogist, Royal Ontario Museum, Toronto, Ontario, Canada.